This invention relates generally to modem technology, more particularly to PCM modems, and, more specifically, to a method of improving echo canceller training in modems and of discovering characteristics of a PCM channel.
The PCM downstream signal is transmitted by the digital PCM modem and received by the analog PCM modem. The analog PCM modem discovers channel impairments experienced by the PCM downstream signal and adapts its receiver to minimize these impairments. As part of the adaptation process, the analog PCM modem sends transmit parameters to the digital PCM modem. The digital PCM modem subsequently uses these parameters when transmitting PCM downstream data.
The impairments experienced by the PCM downstream signal are generally classified as digital and analog channel impairments. The known impairments on the downstream include any combination of the following:
Digital Impairments
A. Digital pads
B. Robbed bit signaling
C. Code conversions
Analog Impairments
A. Sample timing recovery
B. Channel frequency impairments including spectral nulls (DC)
C. Harmonic distortion inversely proportional to frequency
D. Single frequency interference at primarily 50 Hz and 60 Hz
E. Additive receiver noise
Compensation for digital impairments is accomplished by treating the impairments as a set transformation of primary symbols produced by the digital PCM transmitter to secondary symbols represented by the analog voltages on the output of the CO digital to analog converter. This set transformation only preserves the monotonic nature of the PCM symbols. Compensation for the digital impairments involve identification of the primary to secondary set transformation and inhibition of selected transmit symbols such that all primary symbols produce unique secondary symbols. This inhibition can be either alliterative or procedural. The analog PCM modem can send a list of valid and/or invalid primary symbols to the digital PCM modem or a code, such as minimal constellation spacing, which inhibits transmission of certain primary symbols.
Compensation of analog impairments involves primarily training adapti ve components within the analog PCM receiver. Impairments compensated in this fashion include sample timing recovery, channel frequency compensation and single frequency injection. Compensation for spectral nulls, insufficient high frequency response and inversely proportional harmonic distortion is accomplished by creating PCM symbols with a non-white spectral shape. Once the desired transmit spectral shape has been determined; the analog PCM modem sends parameters to the digital PCM modem, which will subsequently result in transmission of the desired spectrum.
Additionally, the presence of receiver noise may require increasing the spacing between PCM transmit symbols. Once discovered, this spacing increase can be considered as a transmit set reduction and can be included in, or combined with, transmit symbol inhibition of digital impairment compensation.
In addition to the compensation and training of the receiver for the digital and analog impairments of the downstream signal, the analog PCM modem receiver traditionally compensates for an echo of the signal it is transmitting. Once in data mode, the analog PCM modem transmitter does not transmit a signal with the same bandwidth as it is receiving. Without the knowledge of the spectral response of the echo outside the bandwidth to which the echo canceller was trained, the echo canceller will interfere with the reception of a wider bandwidth signal such as the downstream PCM signal.
What is needed, then is a method of discovering the characteristics of a PCM channel. What is also needed is a method of echo cancellation that can be trained with a suitable wideband signal in order to train for the wider bandwidth which the receiver is expected to operate with. Use of such a wideband signal allows convergence of the echo canceller over the frequency spectrum comparable to that which it will be receiving in the downstream PCM signal.